Toushirou tantous once enjoyed popularity as both status symbols as well as protection charms, and were highly sought after by many feudal lords. They were also bought and sold by merchants at the time to use for exchange of presents.
Among those, the Toushirou that was obtained by a Hakata merchant would be me! Hakata Toushirou!

Dear Master,
In order to become a big man, I have come to Hakata.
Even though I am a tantou, I have no intention of ending as a small man.
I intend to go back once I have learnt all I can in this fateful place.
But, what should I do after this?

Dear Master,
While I was wandering around, I spotted Lord Kuroda Tadayuki.
Lord Kuroda was one of my previous masters who bought me off from a merchant.
This must be some sort of fate.
Since this is a rare chance, I'll stay with him for a while.

Dear Master,
Lord Kuroda is a person who has a loose grip on his purse. Because of that, he caused a lot of problems.
Although in the end, it might be because of this character of his that made him buy me in the first place.
As I thought, the flow of cash is important. If we lose track of it, it will cause problems for the things that we have to do.
I have decided to learn trade from the merchants. If I do so, it should be of use to everyone.
I hope that you can think of this as an investment, and wait for my return.